In the field of fiber optic technology, there is an increasing interest in the use of large mode area (LMA) fibers, particularly with respect to the fabrication of fiber-based optical amplifiers and the like, since large mode area fibers are known to overcome various nonlinear impairments, such as Raman and Brillouin scattering, thus enabling significant power increases over prior art fibers having conventional core configurations. However, the use of large mode area will increase the presence of fiber-related sensitivities such as macrobend losses, inter-mode coupling and sensitivities to nonuniformities in the fiber's refractive index profile.
There have been at least two different approaches in the prior art to developing LMA optical fibers suitable for high power amplification applications. In one approach, essentially a mechanical approach, rod-like fibers are utilized that are extremely bend resistant. By forcing the fibers to remain essentially straight, both the inter-modal coupling and the bending loss can be significantly reduced. However, in most “field” applications of such fibers, there is a need to bend, even spool, the fiber cables. Therefore, restricting the physical ability of the fiber to bend is considered to be an impractical solution. Another approach is associated with managing bend loss by defining the specific “spooling” to be used, and then always utilizing the fiber in accordance with the specified spooling radius (and number of turns).
While these solutions may be appropriately deployed with conventional fiber designs of moderately large mode area, they have not been found to be effective with LMA fiber, where the least amount of fiber bending (such as required for spooling fiber in the field) has been found to introduce significant distortions (e.g., reduction in the effective area) in the propagating signal mode. Conventional management of bend-induced losses and inter-modal coupling results in a fiber configuration with an effective area substantially reduced by bending.
Thus, a need remains in the art for providing a large mode area fiber whose mode intensity is not seriously distorted as the fiber is subjected to bending in various applications. Moreover, the ability to provide a distortion-resistant mode will result in reduced nonlinear impairment and may also have additional benefits, such as improved interaction of the signal mode with gain materials as compared to prior art distortion-sensitive modes.